This invention pertains to radio frequency generators, and more particularly is concerned with frequency synthesizers.
Frequency hopping synthesizers rapidly shift or "hop" from one transmission carrier frequency to another. They are often used in satellite communication systems where hostile interception or jamming is to avoided.
"Direct" frequency synthesizers are a class of synthesizers in which the output frequency is created without the need for retuning an oscillator. In contrast, the class of synthesizers designated "indirect" utilizes a phase-locked loop with a programmable divide-by-N counter to synthesize an output frequency "indirectly" related to a reference oscillator. The direct synthesizer offers a significant speed advantage in frequency hopping as the need for relocking a phase-locked loop is avoided, thereby allowing faster selection of the frequencies.
Prior to the present invention, direct frequency synthesizers used one of two approaches; (1) an iterative mix-and-divide approach; and (2) a digital look-up table approach.
An iterative mix-and-divide synthesizer contains N mix-and-divide stages connected in series. Each stage, for example, may include a frequency mixer, a 4-to-1 local oscillator selector switch, a fixed upper side band filter and a divide-by-four frequency counter. Four phase locked oscillators generate four LO frequencies or tones, F.sub.l, F.sub.2, F.sub.3, and F.sub.4 which are routed to the 4-to-1 switches which selects one tone to be coupled to the LO port of the corresponding mixer in each of the N identical mix-and-divide stages. The small number of tones are repeatedly mixed to derive an output frequency. It is known to use a bank of four bandpass filters at the output of each mixer to reject spurious signals and leakage. The output frequency is determined by a digital word, 2 bits of which may control the 4-to-1 RF switch and filter selection in each mix-and-divide stage. The need for four bandpass filters results in increased weight, bulk, and cost.
To select one of 2.sup.N frequencies across the programmable bandwidth B, requires N/2 mix-and-divide stages. The programmable bandwidth B is determined by the frequency separation of the four phase locked oscillators such that B=4/3 (F.sub.4 -F.sub.1). The minimum frequency step size is equal to B/2.sup.N. The mix-and-divide synthesizer has the disadvantage of requiring a large amount of stages to produce the desired incremental frequency steps over a large bandwidth. For example, a mix-and-divide synthesizer of 1 GHz bandwidth and 1 Hz incremental step size requires more than 14 mix-and-divide stages (4.sup.14 .times.1Hz=268,435,456 Hz).
A digital look-up table synthesizer on the other hand consists of a parallel register to store a phase increment word, a parallel digital adder, an output parallel register, a phase-to-sine amplitude look-up table, a high speed D/A converter and a low pass filter at the output. In this technique, different frequencies are generated by adding different phase increments with a uniform high speed clock to the digital circuitry. The number of programmable frequencies is a function of the size of the phase increment adder and the number of phase samples desired per output frequency. The minimum number of samples is usually three for spectral purity reasons. The programmable bandwidth is determined by the maximum clock speed of the digital circuitry and D/A converter, the minimum number of phase samples at the highest frequency and the desire to avoid in-band harmonics for the lowest frequency. Thus for a high speed clock frequency f.sub.c, the programmable bandwidth is fc/3-fc/6, or fc/6.
The digital look-up table synthesizer, while much more compact that the mix-and-divide, has more limited programmable bandwidth and poorer spectral purity. This is because current digital and D/A technology limits the high speed clock frequency to less than 300 MHz. The corresponding useable programmable bandwidth is thus 50 MHz or less. The limited word size of the D/A converter introduces white gaussian sampling noise down approximately 48 dB in total power from the power of the desired output sine wave signal. In addition, in-band discrete spurious signals may be as little as 45 dB below the power of the desired output frequency at the output of the D/A converter.